When constructing a power line, it is convenient to use a helicopter for stringing a sock-line which acts as a pilot line to pull through the eventual power line to the suspended position.
In the case of phase lines and top lines which are suspended outside the tower frame, the helicopter can simply lay the sock-lines directly into open line stringing support blocks provided with a closing mechanism to hold the sock-line.
When the sock-line has been positioned within its support blocks on all the towers, the phase line, which is connected to the remote end of the sock-line, is then pulled though the support blocks into place in the usual manner and connected to insulators provided on the tower, and the line stringing support blocks removed.
For those phase lines which are suspended within a closed framed of the tower, for instance underneath the cross member on a portal tower, it is not however possible to string the line directly in this manner since the support cable from the helicopter must pass over the cross-member while the sock-line must pass underneath the cross-member. The sock-line must therefore first be guided into the frame, underneath the cross member, before it can be laid in its line stringing support block.